Method for cleaning semiconductor wafers

ABSTRACT

A method for controlling damages in cleaning a semiconductor wafer comprising features of patterned structures, the method comprising: delivering a cleaning liquid over a surface of a semiconductor wafer during a cleaning process; and imparting sonic energy to the cleaning liquid from a sonic transducer during the cleaning process, wherein power is alternately supplied to the sonic transducer at a first frequency and a first power level for a first predetermined period of time and at a second frequency and a second power level for a second predetermined period of time, the first predetermined period of time and the second predetermined period of time consecutively following one another, wherein at least one of the cleaning parameters is determined such that a percentage of damaged features as a result of the imparting sonic energy is lower than a predetermined threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part application based on PCT InternationalPatent Application Nos. PCT/CN2015/079015 filed May 15, 2015,PCT/CN2015/079342 filed May 20, 2015, PCT/CN2016/078510 filed Apr. 6,2016, PCT/CN2016/099303 filed Sep. 19, 2016, and PCT/CN2016/099428 filedSep. 20, 2016, the entire contents of which are incorporated herein byreference.

FIELD

The present invention generally relates to semiconductor wafer cleaning,and more particularly, to wet cleaning methods and apparatus employingcontrolled sonic energy.

BACKGROUND

Semiconductor devices are manufactured or fabricated on semiconductorwafers employing a sequence of processing steps to create transistorsand interconnection elements. These transistors are traditionally builtin two dimensions but more recently in three dimensions, such as finFETtransistors, as well. The interconnection elements include conductive(e.g., metal) trenches, vias, and the like formed in dielectricmaterials.

In forming these transistors and interconnection elements, semiconductorwafers undergo multiple masking, etching, and deposition processes toform desired structures for the semiconductor devices. For example,multiple masking and plasma etching steps are performed to form recessedareas in a dielectric layer on a semiconductor wafer that serve as finsfor a finFET transistor and trenches and vias for the interconnectionelements. In order to remove particles and contaminations in finstructures and/or trench and via post etching or photoresist ashing, awet cleaning step is necessary. However, a wet cleaning with chemicalsmay result in side wall loss. When device manufacture node migrates downto 14 or 16 nm and beyond, reducing side wall loss in fins, trenches andvias becomes crucial for maintaining critical dimensions. In order toreduce or eliminate the side wall loss, it is important to use moderateor diluted chemicals and sometimes even de-ionized water only. However,the moderate or diluted chemicals or de-ionized water are usually notefficient enough to remove particles in the fin structures and/ortrenches and vias. As a result, mechanical force generated by ultra ormega sonic energy, for instance, is needed in order to remove thoseparticles efficiently. Ultra sonic or mega sonic waves generate bubblecavitation to apply mechanical force to the wafer structures undercleaning.

However, cavitation is a chaotic phenomenon. Onset of cavitation bubbleand its collapse is affected by many physical parameters. A violentcavitation such as transit cavitation or micro jet can damage thosepatterned structures (fins, trenches and vias). In a conventional ultrasonic or mega sonic cleaning process, significant particle removalefficiency (“PRE”) occurs only when the power is high enough, forexample greater than 5-10 watts. However, significant wafer damagesbegin to occur when the power is greater than about 2 watts. Therefore,it is difficult to find a power window where the wafer can be cleanedefficiently without causing significant damages. Therefore, maintaininga stable or controlled cavitation is a key for controlling the sonicmechanical force to be below a damage limit while still being capable ofefficiently removing foreign particles from the patterned structures.

As such, it is desirable to provide a system and method for controllingbubble cavitation generated by ultra or mega sonic devices during awafer cleaning process to be able to efficiently remove fine foreignparticles without damaging patterned structures on the wafer.

SUMMARY

A method for cleaning semiconductor wafers is disclosed which includedelivering a cleaning liquid over a surface of a semiconductor waferduring a cleaning process, imparting sonic energy to the cleaning liquidfrom a sonic transducer during the cleaning process, and alternatelysupplying power to the sonic transducer at a first predetermined settingfor a first predetermined period of time and at a second predeterminedsetting for a second predetermined period of time, wherein bubblecavitation in the cleaning liquid increases during the firstpredetermined period of time and decreases during the secondpredetermined period of time. The first predetermined period of time andthe second predetermined period of time consecutively follow oneanother. Therefore, the bubbles in the cleaning liquid can besufficiently cooled down after the cleaning in each first period of timeto avoid damages to the wafer.

Other aspects, features, and techniques will be apparent to one skilledin the relevant art in view of the following detailed description of theembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings accompanying and forming part of this specification areincluded to depict certain aspects of the present disclosure. A clearerconception of the present disclosure, and of the components andoperation of systems provided with the present disclosure, will becomemore readily apparent by referring to the exemplary, and thereforenon-limiting, embodiments illustrated in the drawings, wherein likereference numbers (if they occur in more than one view) designate thesame elements. The present disclosure may be better understood byreference to one or more of these drawings in combination with thedescription presented herein. It should be noted that the featuresillustrated in the drawings are not necessarily drawn to scale.

FIGS. 1A and 1B illustrate a wafer cleaning apparatus using ultra ormega sonic device according to an embodiment of the present invention.

FIGS. 2A-2G illustrate various shapes of an ultra or mega sonictransducer.

FIG. 3 illustrates bubble implosion during a wafer cleaning process.

FIGS. 4A and 4B illustrate a transit cavitation that damages patternedstructures on a wafer during a wafer cleaning process.

FIGS. 5A-5C illustrate thermal energy variation inside a bubble during asonic wafer cleaning process.

FIGS. 6A-6C illustrate a sonic wafer cleaning process in which a microjet eventually occurs.

FIGS. 7A-7E illustrate a sonic wafer cleaning process according to anembodiment of the present invention.

FIGS. 8A-8D illustrate a sonic wafer cleaning process according toanother embodiment of the present invention.

FIGS. 9A-9D illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention.

FIGS. 10A-10C illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention.

FIGS. 11A-11B illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention.

FIGS. 12A-12B illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention.

FIGS. 13A-13B illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention.

FIGS. 14A-14B illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention.

FIGS. 15A-15D illustrate a stable cavitation that damages patternedstructures on a wafer during a sonic wafer cleaning process.

FIGS. 16A-16C illustrate a wafer cleaning process according to anembodiment of the present invention.

FIG. 17 illustrates a wafer cleaning process according to anotherembodiment of the present invention.

FIGS. 18A-J illustrate bubble cavitation control that enhancescirculation of fresh cleaning liquid in vias or trenches in a wafer.

FIGS. 19A-19D illustrates changes in bubble volume in response to sonicenergy.

FIGS. 20A to 20D illustrates a sonic wafer cleaning process thateffectively cleans high aspect ratio features of vias and trenchesaccording to an embodiment of the present invention.

FIGS. 21A-21C illustrates another cleaning process according anembodiment of the present invention.

FIGS. 22A and 22B illustrate a wafer cleaning process that utilizessonic energy according to another embodiment of the present invention.

FIG. 23 illustrates an exemplary wafer cleaning apparatus for carryingout the wafer cleaning processes illustrated in FIGS. 7-22 according toan embodiment of the present invention.

FIG. 24 is a cross-sectional view of another wafer cleaning apparatusfor carrying out the wafer cleaning processes illustrated in FIGS. 7-22according to an embodiment of the present invention.

FIG. 25 illustrates a control system for monitoring operation parametersof a wafer cleaning process employing sonic energy according to anembodiment of the present invention.

FIG. 26 is a block diagram of the detection system shown in FIG. 25according to an embodiment of the present invention.

FIG. 27 is a block diagram of the detection system shown in FIG. 25according to another embodiment of the present invention.

FIGS. 28A-28C illustrate an exemplary implementation of the voltageattenuation circuit shown in FIG. 26 according to an embodiment of thepresent invention.

FIGS. 29A-29C illustrate an exemplary implementation of the shapingcircuit shown in FIG. 26 according to an embodiment of the presentinvention.

FIGS. 30A-30C illustrate an exemplary implementation of the maincontroller of FIGS. 26 and 27 according to an embodiment of the presentinvention.

FIG. 31 illustrates a sonic power supply that still oscillates severalcycles after the host computer shuts down the sonic power supply.

FIGS. 32A-32C illustrate an exemplary implementation of the amplitudedetection circuit of FIG. 27 according to an embodiment of the presentinvention.

FIG. 33 is a flow chart illustrating a wafer cleaning process accordingto an embodiment of the present invention.

FIG. 34 is a flow chart illustrating a wafer cleaning process accordingto another embodiment of the present invention.

DESCRIPTION

One aspect of the disclosure relates to controlling bubble cavitation insemiconductor wafer cleaning with sonic energy. Embodiments of thepresent disclosure will be described hereinafter with reference to theattached drawings.

FIGS. 1A and 1B illustrate a wafer cleaning apparatus using ultra ormega sonic device according to an embodiment of the present invention.FIG. 1A is a cross-sectional view of the wafer cleaning apparatus thatincludes a wafer chuck 1014 holding a wafer 1010, a rotation drivingmodule 1016 driving the wafer chuck 1014, and a nozzle 1012 deliveringcleaning liquid 1032 to the surface of the wafer 1010. The cleaningliquid 1032 may be cleaning chemicals or de-ionized water. The wafercleaning apparatus also includes an ultra or mega sonic device 1003situated above the wafer 1010, so that with rotation of the wafer 1010and a constant flow of the cleaning liquid 1032 from the nozzle 1012, afilm of the cleaning liquid 1032 with thickness d is maintained betweenthe wafer 1010 and the sonic device 1003. The sonic device 1003 furtherincludes a piezoelectric transducer 1004 acoustically coupled to aresonator 1008 in contact with the cleaning liquid. The piezoelectrictransducer 1004 is electrically excited to vibrate and resonator 1008transmits high frequency sound energy into the cleaning liquid 1032.Bubble cavitation generated by the high frequency sound energy causesforeign particles, i.e., contaminants, on surfaces of the wafer 1010 tovibrate and break loose therefrom.

Referring again to FIG. 1A, the wafer cleaning apparatus also include anarm 1007 coupled to the sonic device 1003 for moving the sonic device1003 in a vertical direction Z, thereby changing the liquid filmthickness d. A vertical driving module 1006 drives vertical movement ofthe arm 1007. Both the vertical driving module 1006 and the rotationdriving module 1016 are controlled by a controller 1088.

Referring to FIG. 1B which is a top view of wafer cleaning apparatusillustrated in FIG. 1A, the sonic device 1003 covers only a small areaof the wafer 1010, which has to rotate to receive uniform sonic energyacross the entire wafer 1010. Although only one such sonic device 1003is illustrated in FIGS. 1A and 1B, in other embodiments, two or moresonic devices may be employed simultaneously or intermittently.Similarly, two or more nozzles 1012 may be employed to deliver thecleaning liquid 1032 more evenly.

FIGS. 2A-2G illustrate various shapes of an ultra or mega sonictransducer. FIG. 2A shows a triangle or pie shape; FIG. 2B shows arectangle shape; FIG. 2C shows an octagon shape; FIG. 2D shows anelliptical shape; FIG. 2E shows a half circle shape; FIG. 2F shows aquarter circle shape; and FIG. 2G shows a full circle shape. Sonictransducers in each of these shapes may be used in place of thepiezoelectric transducer 1004 in the sonic device 1003 shown in FIG. 1.

FIG. 3 illustrates bubble implosion during a wafer cleaning process. Theshape of a bubble 3052 is gradually compressed from a spherical shape Ato an apple shape G as sonic energy is applied to the bubble 3052.Finally the bubble 3052 reaches to an implosion status I and forms amicro jet. As shown in FIGS. 4A and 4B, the micro jet is very violent(can reach a few thousand atmospheric pressures and a few thousand °C.), which can damage the fine patterned structure 4034 on the wafer4010, especially when the feature size t shrinks to 70 nm and smaller.

FIGS. 4A and 4B illustrate a transit cavitation that damages patternedstructures on a wafer during a wafer cleaning process. Referring to FIG.4A, bubbles 4040, 4042 and 4044 are formed by sonic cavitation over apatterned structure 4034 on a semiconductor wafer 4010. The patternedstructure 4034 comprises a plurality of features that need to becleaned, including but not limited to fins, vias, trenches, etc. Thebubble 4044 is turned into a micro jet which can be very violent,reaching a few thousand atmospheric pressures and a few thousand degreesCelsius. Referring to FIG. 4B, once the micro jet occurs, a portion ofthe patterned structure 4034 is blown away. Such damage is more acutefor wafers with device feature size of 70 nm and below.

FIGS. 5A-5C illustrate thermal energy variation inside a bubble 5016during a wafer cleaning process. As sonic positive pressure acting onthe bubble 5106, the bubble 5106 reduces its volume as shown in FIG. 5A.During this volume reduction process, the sonic pressure P_(M) forces onthe bubble 5016, and the mechanical work converts to thermal energyinside the bubble 5016. Therefore, temperature T of gas and/or vaporinside the bubble 5016 increases as shown in FIG. 5B. Relationshipbetween various parameters can be expressed by the following equation:

p ₀ v ₀ /T ₀ =pv/T   (1)

where p₀ is a pressure inside the bubble before compression, v₀ is aninitial volume of the bubble 5016 before compression, T₀ is atemperature of gas inside the bubble before compression, p is a pressureinside the bubble during compression, v is a volume of the bubble duringcompression, and T is a temperature of gas inside the bubble duringcompression.

In order to simplify the calculation, we may assume the temperature ofgas does not change during the compression or the compression is veryslow and temperature increase is cancelled by liquid surrounding thebubble. So the mechanical work w_(m) caused by sonic pressure P_(M)during one time of bubble compression (from volume N unit to volume 1unit, or compression ratio=N) can be expressed as follows:

$\begin{matrix}{w_{m} = {{\int_{0^{x^{0_{- 1}}}}^{\;}{pSdx}} = {{\int_{0^{x^{0_{- 1}}}}^{\;}{\left( {{S\left( {x_{0}p_{0}} \right)}/\left( {x_{0} - x} \right)} \right){dx}}} = {{{Sx}_{0}p_{0}{\int_{0^{x^{0_{- 1}}}}^{\;}{{dx}/\left( {x_{0} - x} \right)}}} = {\left. {{- {Sx}_{0}}p_{0}{\ln \left( {x_{0} - x} \right)}} \right|^{0^{x^{0_{- 1}}}} = {{Sx}_{0}p_{0}{\ln \left( x_{0} \right)}}}}}}} & (2)\end{matrix}$

where S is an area of cross section of a cylinder, x₀ is a length of thecylinder, p₀ is a pressure of gas inside the cylinder before thecompression. Equation (2) does not consider the factor of temperatureincrease during the compression, so that the actual pressure inside thebubble will be higher due to temperature increase. Therefore the actualmechanical work by sonic pressure will be larger than the valuecalculated by equation (2).

Assuming the mechanical work by sonic pressure is partially converted tothermal energy and partially converted mechanical energy of highpressure gas and/or vapor inside the bubble, and such thermal energy isfully contributed to temperature increase of gas inside the bubble (noenergy is transferred to liquid molecules surrounding the bubble), andassuming the mass of gas inside the bubble stays constant before andafter the compression, a temperature increase AT after one time ofcompression of bubble can be expressed by the following formula:

ΔT=Q/(mc)=βw _(m)/(mc)=βSx ₀ p ₀ ln(x ₀)/(mc)   (3)

where Q is thermal energy converted from mechanical work, β is a ratioof thermal energy to total mechanical work by sonic pressure, m is amass of gas inside the bubble, c is a specific heat coefficient of thegas. If β=0.65, S=1E-12 m2, x₀=1000 μm=1E-3 m (compression ratioN=1000), p₀=1 kg/cm2=1E4 kg/m2, m=8.9E-17 kg for hydrogen gas, c=9.9E3J/(kg ° k), then ΔT=50.9° C.

The temperature T₁ of gas inside the bubble after the first compressioncan be calculated as:

T ₁ =T ₀ +ΔT=20° C.+50.9° C.=70.9° C.   (4)

when the bubble reaches the minimum size of 1 micron as shown in FIG.5B. At such a high temperature, some liquid molecules surrounding bubblewill evaporate. After that, the sonic pressure becomes negative and thebubble starts to increase its size. In this reverse process, the hot gasand/or vapor with pressure P_(G) will do work to the surrounding liquidsurface. At the same time, the sonic pressure P_(M) is pulling bubble toexpansion direction as shown in FIG. 5C. Therefore the negative sonicpressure P_(M) also does partial work to the surrounding liquid. As aresult of the joint efforts, the thermal energy inside the bubble cannotbe fully released or converted to mechanical energy, therefore thetemperature of gas inside bubble cannot cool down to the original gastemperature T₀ or to the liquid temperature. After the first cycle ofcavitation, the temperature T₂ of gas and/or vapor inside the bubblewill be somewhere between T₀ and T₁ as shown in FIG. 6B. Here, T₂ can beexpressed as:

T ₂ =T ₁ −δT=T ₀ +ΔT−δT   (5)

where δT is a temperature decrease after one time of expansion of thebubble, and δT is smaller than ΔT.

When a second cycle of bubble cavitation reaches the minimum bubblesize, the temperature T₃ of gas and/or vapor inside the bubble will be:

T ₃ =T ₂ +ΔT=T ₀ +ΔT−δT+ΔT=T ₀+2ΔT−δT   (6)

When the second cycle of bubble cavitation finishes, the temperature T₄of gas and/or vapor inside the bubble will be:

T ₄ =T ₃ −δT=T ₀+2ΔT−δT−δT=T ₀+2ΔT−2δT   (7)

Similarly, when the nth cycle of bubble cavitation reaches the minimumbubble size, the temperature T_(2n−1) of gas and/or vapor inside thebubble will be:

T _(2n−1) =T ₀ +nΔT−(n−1)δT   (8)

When the nth cycle of bubble cavitation finishes, the temperature T2n ofgas and/or vapor inside the bubble will be:

T _(2n) =T ₀ +nΔT−nδT=T ₀ +n(ΔT−δT)   (9)

From equation (8), implosion cycle number n_(i) can be written asfollows:

n _(i)=(T _(i) −T ₀ −ΔT)/(ΔT−δT)+1   (10)

From equation (10), implosion time τ_(i) can be written as follows:

$\begin{matrix}{{\tau_{i} = {{n_{i}t_{1}} = {{t_{1}\left( {{\left( {T_{i} - T_{0} - {\Delta \; T}} \right)/\left( {{\Delta \; T} - {\delta \; T}} \right)} + 1} \right)} = {{n_{i}/f_{1}} = {\left( {{\left( {T_{i} - T_{0} - {\Delta \; T}} \right)/\left( {{\Delta \; T} - {\delta \; T}} \right)} + 1} \right)f_{1}}}}}}\;} & (11)\end{matrix}$

where t₁ is a cycle period, and f₁ is a frequency of ultra/mega sonicwave.

Based on equations (10) and (11), implosion cycle number n_(i) andimplosion time τ_(i) can be calculated. Table 1 shows calculatedrelationships among implosion cycle number n_(i), implosion time τ_(i)and (ΔT−δT), assuming Ti=3000° C., ΔT=50.9° C., T0=20° C., and f1=500KHz, 1 MHz, or 2 MHz.

TABLE 1 ΔT − δT (° C.) 0.1 1 10 30 50 n_(i) 29018 2903 291 98 59 τ_(i)(ms) 58.036 5.806 0.582 0.196 0.118 f₁ = 500 KHz τ_(i) (ms) 29.018 2.9030.291 0.098 0.059 f₁ = 1 MHZ τ_(i) (ms) 14.509 1.451 0.145 0.049 0.029|f₁ = 2 MHz

FIGS. 6A-6C illustrate a sonic wafer cleaning process in which a microjet eventually occurs and processing parameters adhere to equations(1)-(11). Referring to FIG. 6A, electric power (P) is continuouslysupplied to a sonic device to generate bubble cavitation in a cleaningliquid. As cycle number n of bubble cavitation increases, thetemperature of gas and/or vapor will increase as shown in FIG. 6B, thusmore molecules on bubble surface will evaporate into inside of a bubble6082, resulting in its size increase over time as shown in FIG. 6C.Finally the temperature inside the bubble 6082 during compression willreach implosion temperature Ti (normally Ti is as high as a few thousand° C.), and violent micro jet 6080 occurs as shown in FIG. 6C. Therefore,in order to avoid damages to patterned structures of wafer duringcleaning, a stable cavitation must be maintained, and bubble implosionor micro jet must be avoided.

FIGS. 7A-7E illustrate a sonic wafer cleaning process according to anembodiment of the present invention. FIG. 7A shows a waveform of powersupply output which is intermittently supplied to a sonic device togenerate bubble cavitation in a cleaning liquid. FIG. 7B shows atemperature curve corresponding to each cycle of the cavitation. FIG. 7Cshows that during each cycle of cavitation, bubble size increases in aτ₁ time period and decreases when power supply is terminated in a τ₂time period.

Detailed processing steps to avoid bubble implosion according to a firstembodiment of the present invention are illustrated in FIG. 7D. Theprocessing steps begins with step 7010 in which an ultra or mega sonicdevice is placed near an upper surface of a wafer under cleaning. Instep 7020, a cleaning liquid, either chemicals or gas doped water, isinjected over the wafer to fill a gap between the wafer and the sonicdevice. In step 7030, the wafer carried by a chuck starts to rotate oroscillate. In step 7040, a power supply with a frequency of f1 and apower level P1 is applied to the sonic device. In step 7050, beforetemperature of gas and/or vapor inside a bubble reaches implosiontemperature Ti, or before time τ₁ reaches τ_(i) as calculated byequation (11), power supply output is set to zero, therefore thetemperature of gas and/or vapor inside the bubble starts to cool downsince the temperature of the cleaning liquid is much lower than the gastemperature. In step 7060, after the temperature of gas and/or vaporinside the bubble decreases to room temperature T₀ or time durationreaches τ₂ (during time period τ₂, the power supply output is set tozero), power supply output is restored to frequency f1 and power levelP1. In step 7070, the wafer's cleanliness is inspected, and steps7010-7060 are repeated if the wafer is not yet cleaned to a desireddegree. Alternatively, inspection of cleanliness may not be performedfor every cycle. Instead, the number of cycles to be used maybeempirically determined beforehand using a sample wafer.

Referring to FIG. 7D again, in step 7050, the time period τ₁ must beshorter than τ_(i) in order to avoid bubble implosion, where τ_(i) canbe calculated by using equation (11). In step 7060, the temperature ofgas and/or vapor inside the bubble does not need to be cooled down toroom temperature or cleaning liquid temperature. Rather, it can be acertain temperature above room temperature or cleaning liquidtemperature. Preferably the temperature is significantly lower thanimplosion temperature Ti.

According to equations (8) and (9), if (ΔT−δT) is known, then theimplosion time τ_(i) can be calculated. But in general, (ΔT−δT) cannotbe calculated or directly measured easily. However, τ_(i) can bedetermined empirically.

FIG. 7E is a flow chart illustrating steps for empirically determiningthe implosion time τ_(i). In step 7210, 5 different time periods τ₁ areexemplarily chosen as design of experiment (DOE) conditions based onTable 1. In step 7220, a time period τ₂ is set at least 10 times longerthan the chosen time period τ₁, and preferably 100 times longer in afirst screening test. In step 7230, the power supply level is fixed atP₀ to run at the above five DOE conditions to separately clean 5different wafers with the same specific patterned structure. Here, P₀ isa power level at which the patterned structure will certainly be damagedwhen running on continuous mode (non-pulse mode) as shown in FIG. 6A. Instep 7240, damage status of the 5 wafers are inspected by scanningelectron microscope (SEM) or wafer pattern damage review tool such asAMAT SEM vision or Hitachi IS3000, so that the implosion time τ_(i) canbe narrowed to a certain range. A percentage of damaged features can becalculated by dividing the total number of damaged features as inspectedby the SEM by the total number of features of the patterned structure.There may be other ways for determining the percentage of damagedfeatures. For example, the final wafer yield may be used as anindication of the percentage of damaged features.

The above steps 7210 through 7240 can be repeated to narrow down therange of implosion timeτ_(i). After knowing the implosion time τ_(i),the time τ₁ can be set at a value smaller than 0.5*τ_(i) to allow asafety margin. The following paragraph describes an example of suchexperiment.

Suppose a patterned structure is formed by 55 nm poly-silicon gatelines; ultra sonic wave frequency is 1 MHz generated by a ultra/megasonic device manufactured by Prosys operating in a gap oscillation mode(disclosed in PCT Application No. PCT/CN2008/073471) for achieving auniform energy dose within wafer and from wafer to wafer. Otherexperimental parameters and final pattern damage data are summarized inTable 2 as follows:

TABLE 2 CO₂ Power Number conc. Process Density of Wafer (18 Time (Watts/Cycle τ₁ τ₂ Damage ID μs/cm) (sec) cm2) Number (ms) (ms) Sites #1 18 600.1 2000 2 18 1216 #2 18 60 0.1 100 0.1 0.9 0

In an experiment, when τ_(i)=2 ms (or 2000 cycles), the aforementionedsonic cleaning process introduces as many as 1216 damage sites to thepatterned structure with 55 nm feature size. When τ₁=0.1 ms (or 100cycles), the sonic cleaning process introduces zero (0) damage sites tothe same patterned structure. So that the τ_(i) is a time value between0.1 ms and 2 ms. Additional tests with narrowed τ_(i) range can yield anarrower τ_(i) range.

In the above experiment, the cycle number depends on ultra or mega sonicpower density and frequency: the larger the power density, the less thecycle number is; and the lower the frequency, the less the cycle numberis. From the above experiments, a damage-free cycle number can bepredicted to be smaller than 2,000 given the power density of ultra ormega sonic wave is larger than 0.1 watts/cm², and the frequency of ultraor mega sonic wave is equal to or less than 1 MHz. If the frequencyincreases to a range larger than 1 MHz or the power density is less than0.1 watts/cm², it can be predicted that the cycle number will increase.

After acquiring the time period τ₁, the time period τ₂ can beempirically obtained based on similar DOE method as described above. Inthis case τ₁ is fixed at a predetermined value, and τ₂ is graduallyshortened in each DOE run until damage on patterned structure isobserved. As the time period τ₂ is shortened, the temperature of gasand/or vapor inside bubble cannot be cooled down enough, which willgradually increase the average temperature of gas and/or vapor insidethe bubble, and eventually trigger an implosion of the bubble. Thistrigger time is called critical cooling time τ_(c). With knowledge ofthe critical cooling time τ_(c), the time period τ₂ can be set at avalue larger than 2*τ_(c) to allow a safety margin.

Therefore, parameters of the cleaning process may be determined suchthat a cleaning effect of imparting the sonic energy causes a yieldimprovement greater than a yield degradation caused by damages as aresult of imparting the sonic energy. A predetermined threshold for thepercentage of damages may also be specified, for example by a customer.Parameters of the cleaning process may be determined such that thepercentage of damages is lower than the predetermined threshold, orsubstantially zero, or even zero. The predetermined threshold, forexample, may be 10%, 5%, 2%, or 1%. The percentage of damages issubstantially zero if the final yield of wafer production is notsubstantially impacted by any damages caused by the cleaning process. Inother words, any damages caused by the cleaning process are tolerable inview of the entire manufacturing process. The percentage of damages canbe determined by inspecting a sample wafer using electron microscopy, asdiscussed above.

FIGS. 8A-8D illustrate a sonic wafer cleaning process according toanother embodiment of the present invention. In the present sonic wafercleaning process, amplitude of the power supply P, instead of beingmaintained at a constant level P1 as shown FIG. 7A and in step 7040 ofFIG. 7D, varies over time while other aspects of the process remain thesame as the one shown in FIGS. 7A-7D. In one embodiment, as shown inFIG. 8A, the power supply amplitude P increases during the time periodτ₁. In another embodiment, as shown in FIG. 8B, the power supplyamplitude P decreases during the time period τ₁. In yet anotherembodiment, as shown in FIG. 8C, the power supply amplitude P decreasesfirst and then increases during the time period τ₁. In an embodimentshown in FIG. 8D, the power supply amplitude P increases first and thendecreases during the time period τ₁.

FIGS. 9A-9D illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention. In the present sonic wafercleaning process, frequency of the power supply, instead of beingmaintained at a constant f1 as shown FIG. 7A and in step 7040 of FIG.7D, varies over time while other aspects of the cleaning process remainthe same as the one shown in FIGS. 7A-7D. As shown in FIG. 9A, in oneembodiment the power supply frequency is first set at f1 and then at f3,where f1 is higher than f3, during the time period τ₁. As shown in FIG.9B, in one embodiment the power supply frequency is first set at f3 andlater increased to f1 during the time period τ₁. As shown in FIG. 9C, inone embodiment the power supply frequency changes from f3 to f1 and thenback to f3 during the time period τ₁. As shown in FIG. 9D, in oneembodiment the power supply frequency changes from f1 to f3 and thenback to f1 during the time period τ₁.

FIGS. 10A-10C illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention. Referring to FIG. 10A,similar to the cleaning process shown in FIG. 7A, during time period τ₁,a power supply with a level of P1 and frequency of f1 is applied to thesonic device. However, during time period τ₂, the power supply, insteadof dropping to zero as shown in FIG. 7A, decreases to a level of P2. Asa result, the temperature of gas and/or vapor inside the bubblesdecreases to T₀+ΔT₂ as shown in FIG. 10B.

FIG. 10C is a flow chart illustrating steps of the wafer cleaningprocess shown in FIGS. 10A and 10B. In step 10010, an ultra or megasonic device is placed near an upper surface of a wafer under cleaning.In step 10020, a cleaning liquid, either chemicals or gas doped water,is injected over the wafer to fill a gap between the wafer and the sonicdevice. In step 10030, a chuck carrying the wafer starts to rotate forthe cleaning process. In step 10040, a power supply with a frequency off1 and a power level P1 is applied to the sonic device. In step 10050,while maintaining the frequency at f1, the power supply level is loweredto P2 before temperature of gas and/or vapor inside the bubble reachesimplosion temperature τ_(i), or before time τ₁ reaches τ_(i) ascalculated by equation (11). In step 10060, the power supply level isrestored to P1 after the temperature of gas and/or vapor inside thebubble decreases to close to room temperature T₀ or time durationreaches τ₂. In step 10070, the wafer cleanliness is inspected, and steps10010-10060 will be repeated if the wafer is not yet cleaned to adesired degree. Alternatively, inspection of cleanliness may not beperformed for every cycle. Instead, the number of cycles to be usedmaybe empirically determined beforehand using a sample wafer.

FIGS. 11A-11B illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention. The present sonic wafercleaning process is similar to the one shown in FIGS. 10A-10C, withdifferences existing only in step 10050. Instead of maintaining thepower supply frequency at f1, the wafer cleaning process shown in FIGS.11A and 11B lowers the frequency to f2 during time period τ₂. The powerlevel P2 should be significantly less than P1, preferably 5 or 10 timesless, in order to allow the temperature of gas and/or vapor inside thebubble to be lowered to close to the room temperature T₀.

FIGS. 12A-12B illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention. Differences between thepresent cleaning process and the one shown in FIGS. 10A-10C are only instep 10050. In the present wafer cleaning process, the power supplyfrequency is increased to f2 while the power supply level P2 issubstantially equal to P1 during time period τ₂.

FIGS. 13A-13B illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention. Differences between thepresent cleaning process and the one shown in FIGS. 10A-10C are only instep 10050. In this wafer cleaning process, the power supply frequencyis increased to f2 while the power supply level is lowered from P1 to P2during time period τ₂.

FIGS. 14A-14B illustrate a sonic wafer cleaning process according to yetanother embodiment of the present invention. Differences between thepresent cleaning process and the one shown in FIGS. 10A-10C are alsoonly in step 10050. In the present wafer cleaning process, the powersupply frequency is increased from f1 to f2 while the power supply levelis also increased from P1 to P2 during time period τ₂. Since thefrequency f2 is higher than f1, hence the sonic energy heats up thebubble less intensely, the power supply level P2 can be slightly higherthan P1, but must not be too high to ensure that the temperature of gasand/or vapor inside the bubble decreases during time period τ₂ as shownin FIG. 14B.

FIGS. 15A-15C illustrate a stable cavitation that damages patternedstructures on a wafer during a sonic wafer cleaning process. Referringto FIG. 15A, a patterned structure 15034 with a spacing W is formed on awafer 15010. Some bubbles 15046 formed in a cavitation process areinside the space of the patterned structure 15034. Referring to FIG.15B, as bubble cavitation continues, temperature of gas and/or vaporinside the bubbles 15048 increases, which causes the sizes of thebubbles 15048 to increase. When the size of the bubbles 15048 becomelarger than the spacing W, the expansion force of the bubble cavitationcan damage the pattern structure 15034 as shown in FIG. 15C. Therefore,a new wafer cleaning process is needed.

A damage site caused by bubble expansion, as illustrated in FIG. 15C,may be smaller than a damage site caused by bubble implosion, asillustrated in FIG. 4B. For example, bubble expansion may result in adamage site in the order of magnitude of 100 nm, while bubble implosionmay result in a larger damage site in the order of magnitude of 1 μm.

FIG. 15D is a flow chart illustrating an alternative wafer cleaningprocess according to an embodiment of the present invention. Thealternative wafer cleaning process begins with step 15210 in which anultra or mega sonic device is placed near an upper surface of a waferunder cleaning. In step 15020, a cleaning liquid, either chemicals orgas doped water, is injected over the wafer to fill a gap between thewafer and the sonic device. In step 15230, the wafer carried by a chuckstarts to rotate or oscillate. In step 15240, a power supply with afrequency of f1 and a power level P1 is applied to the sonic device. Instep 15250, before sizes of bubbles reach the value of the spacing W,the power supply output is set to zero, so that the temperature of gasand/or vapor inside the bubble starts to cool down, as the temperatureof the cleaning liquid is much lower than the gas temperature. In step15260, after the temperature of gas and/or vapor inside the bubbledecreases to room temperature T₀ or time duration reaches τ₂ (duringtime period τ₂, the power supply output is set to zero), power supplyoutput is restored to frequency f1 and power level P1. In step 15270,the wafer's cleanliness is inspected, and steps 15210-15260 are repeatedif the wafer is not yet cleaned to a desired degree. Alternatively,inspection of cleanliness may not be performed for every cycle. Instead,the number of cycles to be used maybe empirically determined beforehandusing a sample wafer.

Referring again to FIG. 15D, the temperature of gas and/or vapor insidethe bubbles does not need to be cooled down to the room temperature T₀,but preferably should be cooled down to much lower than the implosiontemperature Ti. In addition, in step 15250, the sizes of the bubbles canbe slightly larger than the spacing W of the patterned structure 15034as long as bubble expansion force does not break or damage the patternedstructure 15034.

Referring again to FIG. 15D, the time duration of step 15240 can beempirically obtained as τ₁ from the procedure illustrated in FIG. 7E. Insome embodiments, the wafer cleaning processes illustrated in FIGS. 7-14can be combined with the wafer cleaning process illustrated in FIG. 15.

FIGS. 16A-16C illustrate a wafer cleaning process according to anembodiment of the present invention. This wafer cleaning process issimilar to the one shown in FIGS. 7A-7E except in step 7050 of FIG. 7D.This wafer cleaning process sets power supply output to a positive DCvalue shown in FIG. 18A or a negative DC value shown in FIGS. 18B and18C before temperature of gas and/or vapor inside the bubble reachesimplosion temperature Ti, or time duration τ₁ reaches τ_(i) ascalculated by equation (11). As a result, the temperature of gas and/orvapor inside the bubble starts to decrease as the temperature of thecleaning liquid is much lower than the gas and/or vapor temperature. Insome embodiments, the amplitude of the DC output, either positive ornegative, can be larger (not shown), equal to (shown in FIGS. 16A) and16B) or smaller (shown in FIG. 16C) than the amplitude of the powersupply level P1 which is applied during the time period τ₁ for creatingbubble cavitation in the cleaning liquid.

FIG. 17 illustrates a wafer cleaning process according to anotherembodiment of the present invention. This wafer cleaning process is alsosimilar to the one shown in FIGS. 7A-7E except in step 7050 of FIG. 7D.This wafer cleaning process reverses phase of the power supply outputwhile maintaining the same frequency f1 as applied during the timeperiod τ₁, so that the bubble cavitation can be quickly stopped. As aresult, the temperature of gas and/or vapor inside the bubble starts todecrease, as the temperature of the cleaning liquid is much lower thanthe gas and/or vapor temperature.

Referring to FIG. 17 again, the power supply level during the timeperiod τ₂ is P2 which can be, in different embodiments, larger, equal toor less than P1 which is the power supply level during the time periodτ₁. In an embodiment, the power supply frequency during time period τ₂can be different from f1 as long as the phase is reversed. In someembodiments, the ultra or mega sonic power supply frequency f1 isbetween 0.1 MHz to 10 MHz.

FIGS. 18A-J illustrate bubble cavitation control that enhancescirculation of fresh cleaning liquid in vias or trenches in a wafer.FIG. 18A is a cross-sectional view of a plurality of vias 18034 formedin a wafer 18010. A diameter of the via opening is denoted as W1.Bubbles 18012 generated by sonic energy in the vias 18034 enhancesremoval of impurities such as residues and particles therefrom. FIG. 18Bis a top view of the vias shown in FIG. 18A.

FIG. 18C is a cross-sectional view of a plurality of trenches 18036formed in the wafer 18010. Similarly, bubbles 18012 generated by sonicenergy in the trenches 18036 enhances removal of impurities such asresidues and particles therefrom. FIG. 18D is a top view of the trenches18036 shown in FIG. 18C.

A saturation point Rs is defined by the largest amount of bubbles thatcan be contained inside features of the vias 18034, the trench 18036 oranother recessed area. When the amount of bubble is over the saturationpoint Rs, cleaning liquid will be blocked by the bubbles and can hardlyreach to the bottom of side walls of the feature of the via 18034 or thetrench 18036, so that cleaning performance will be negatively affected.When the amount of bubble is below the saturation point, the cleanliquid will have ample availability inside the features of the via 18034or the trench 18036, hence a good cleaning performance can be achieved.

Below the saturation point, the ratio R of total bubble volume V_(B) tothe volume of vias or trenches, or recessed spaces V_(VTR) is:

R=V _(B) /V _(VTR) <Rs

And at the saturation point Rs, the ratio R is

R=V _(B) /V _(VTR) =Rs

The volume of the total bubbles in the features of the vias 18034,trenches 18036 or other recessed space is:

V _(B) =N*V _(B)

Wherein N is a number of bubbles in the features and V_(B) is an averagevolume of a single bubble.

As shown in FIGS. 18E-18H, when ultra or mega sonic energy is applied tothe cleaning liquid, sizes of the bubbles 18012 expands gradually to acertain volume, which causes the ratio R of total bubble volume V_(B) tothe volume of vias, trenches or recessed spaces V_(VTR) to be close toor above the saturation point Rs. The expanded bubbles 18012 block thepath of cleaning liquid exchanges and impurities removal in the vias ortrenches. In this case, the sonic energy cannot efficiently transferinto the vias or trenches to reach their bottoms and sidewalls, whilethe particles, residues and other impurities 18048 are trapped in thevias or trenches. This case can easily occur in advanced semiconductorprocesses as the critical dimension W1 becomes smaller.

As shown in FIG. 18I to FIG. 18J, size expansion of the bubbles 18012 bythe ultra or mega sonic energy is within a limit, and the ratio R oftotal bubble volume V_(B) to the volume of vias, trenches or recessedspaces V_(VTR) is much lower than the saturation point Rs. Freshcleaning liquid 18047 circulates freely in the vias or trenches due tosmall bubble cavitation inside the features, so that the impurities18048, such as residues and particles, can be forced out of the featureswith ease for a good cleaning performance.

Because the total volume of bubbles in a feature of via or trench isdetermined by the number and the sizes of the bubbles, controlling thebubble size expansion due to cavitation is critical for the cleaningperformance for a wafer with high aspect ratio features.

FIGS. 19A-19D illustrate changes in bubble volume in response to sonicenergy. During a first cycle of cavitation, a volume of a bubble iscompressed from V₀ to V₁ after the positive sonic power cycle, andexpands to V₂ after the negative sonic power cycle. However, temperaturein the bubble T₂ corresponding to V₂ is higher than temperature T₀corresponding to V₀, so that the volume V₂ is bigger than the volume V₀as shown in FIG. 19B. This volume increase is caused by liquid moleculessurrounding the bubble being evaporated under the higher temperature.Similarly, the volume of V₃ after the second compression of the bubbleis somewhere between V₁ and V₂, as shown in FIG. 21B. And V₁, V₂ and V₃can be expressed as

V ₁ =V ₀ −ΔV   (12)

V ₂ =V ₁ +δV   (13)

V ₃ =V ₂ −ΔV=V ₁ +δV−ΔV=V ₀ −ΔV+δV−ΔV=V ₀ +δV−2ΔV   (14)

where ΔV is a volume compression of the bubble after one compression dueto positive pressure generated by ultra/mega sonic wave, and δV is avolume increase of the bubble after one expansion due to negativepressure generated by ultra/mega sonic wave, and (δV−ΔV) is volumeincrease due to temperature increase (ΔT−δT) as calculated in equation(5) after one time cycle.

After the second cycle of bubble cavitation, the bubble expands to alarger size while the temperature keeps increasing. The volume of V₄ ofgas and/or vapor inside the bubble will be

V ₄ =V ₃ +δV=V ₀ +δV−2ΔV+δV=V ₀+2(δV−ΔV)   (15)

After the third compression, the volume V₅ of gas and/or vapor insidethe bubble will be

V ₅ =V ₄ −ΔV=V ₀+2(δV−ΔV)−ΔV=V ₀+2δV−3ΔV   (16)

Following this pattern, when the nth cycle of bubble cavitation reachesthe minimum bubble size, the volume V2n-1 of gas and/or vapor inside thebubble will be

V _(2a−1) =V ₀+(n−1)δV−nΔV=V ₀+(n−1)δV−nΔV   (17)

When the nth cycle of bubble cavitation finishes, the volume V_(2n) ofgas and/or vapor inside the bubble will be

V _(2n) =V ₀ +n(δV−ΔV)   (18)

To limit the volume of bubble to a desired volume Vi, which is adimension with enough physical movement feasibility or the status belowthe saturation point, and prevent blocking of the path of cleaningliquid exchange in the features of vias, trenches or other recessedareas, the cycle number n_(i) can be written as follows:

n _(i)=(V _(i) −V ₀ −ΔV)/(δV−ΔV)+1   (19)

From equation (19), a desired time τ_(i) to achieve Vi can be written asfollows:

$\begin{matrix}{{\tau_{i} = {{n_{i}t_{1}} = {{t_{1}\left( {{\left( {V_{i} - V_{0} - {\Delta \; V}} \right)/\left( {{\delta \; V} - {\Delta \; V}} \right)} + 1} \right)} = {{n_{i}/f_{1}} = {\left( {{\left( {V_{i} - V_{0} - {\Delta \; T}} \right)/\left( {{\delta \; V} - {\Delta \; V}} \right)} + 1} \right)f_{1}}}}}}\;} & (20)\end{matrix}$

where τ₁ is a cycle period, and f₁ is a frequency of ultra/mega sonicwave. Therefore the desired cycle number n_(i) and the desired timeτ_(i) for preventing the bubble dimension from reaching a featureblocking level can be calculated from equations (19) and (20).

Note that when the cycle number n of bubble cavitation increases,temperature of gas and/or vapor inside the bubble will increase,therefore more molecules on the bubble surface will evaporate into theinside of the bubble. Therefore the size of the bubble 19082 willfurther increase and become bigger than value calculated by equation(18). In operation, since the bubble size will be determined byexperimental method to be disclosed hereinafter, bubble size impacted bythe evaporation of liquid or water into the bubble inner surface due totemperature increase will not be theoretically discussed in detail here.As the average single bubble volume keeps increasing, the ratio R oftotal bubbles volume V_(B) to the volume of vias, trenches or otherrecessed spaces V_(VTR) increases from R0 continuously as shown in FIG.19D.

As the bubble volume increases, the diameters of the bubbles eventuallywill reach the same size or same order in size of the feature W1 of thevia 18034 as shown in FIGS. 18A and 18B or the trench 18036 as shown inFIGS. 18C and 18D. Then the bubbles inside the via 18034 and the trench18036 will block ultra/mega sonic energy from further getting into thebottom thereof, especially when the aspect ratio (depth/width) is 3 ormore. Therefore contaminations or particles at the bottom of such a deepvia or trench cannot be effectively removed or cleaned. Therefore, a newcleaning processing is proposed to prevent the bubble from growing up toa critical dimension to block the path of cleaning liquid exchanges inthe features of vias or trenches.

FIGS. 20A to 20D illustrates a sonic wafer cleaning process toeffectively clean high aspect ratio features of vias and trenchesaccording to an embodiment of the present invention. This wafer cleaningprocess limits the size of bubbles in cavitation by sonic energy. FIG.20A shows a waveform of power supply output where the power level is setat P1 during a time period τ₁ and turn off during a time period τ₂. FIG.20B shows the bubble volume curve corresponding to each cycle ofcavitation. FIG. 20C shows the bubble size expansion during each cycleof cavitation. FIG. 20D shows the curve of the ratio R of total bubblevolume V_(B) to the volume of via, trench or other recessed spaceV_(VTR). According to

R=V _(B) /V _(VTR) =Nv _(b) /V _(VTR)

where the ratio R of total bubble volume V_(B) to the volume of via,trench or recessed space V_(VTR) increases from R₀ to R_(n), where theaverage single bubble volume being expanded by the sonic cavitationafter a certain cycle number n, in the time τ₁. And R_(n) is controlledbelow the saturation point R_(s),

R _(n) =V _(B) /V _(VTR) =Nv _(b) /V _(VTR) <Rs.

And the ratio R of total bubble volume V_(B) to the volume of via,trench or other recessed space V_(VTR) decreases from R_(n) to R₀, wherethe average single bubble volume return to the original size in thecooling process in the time τ₂.

Referring to FIG. 20B again, the bubble is expanded into a large volumeVn under the ultra/mega sonic power applied to the cleaning liquidduring a time τ₁. At this state, the path of mass transfer is partiallyblocked. Then fresh cleaning liquid cannot thoroughly flow into thebottom and sidewall of vias or trenches. Meanwhile, particles, residuesand other impurities trapped in the vias and trenches cannot be removedefficiently. But this state will alternate into the next state of bubbleshrinking when the ultra/mega sonic power is turned off for cooling thebubble during a time τ₂ as shown in FIG. 20A. In this cooling state,fresh cleaning liquid has a chance to flow into the vias and trenchesfor cleaning the bottom and sidewall thereof. When the ultra/mega sonicpower is turned on again in the next cycle, the particles, residues andother impurities can be removed from the vias and trenches bypulling-out force generated by bubble volume increase. When the twostates alternate in a cleaning process employing ultra/mega sonic wave,high aspect ratio features of vias, trenches and other recessed areas ona wafer substrate can be effectively cleaned.

The cooling state in the time τ₂ plays a key role in this cleaningprocess. And a condition, τ₁<τ_(i), to restrict bubble size, is desired.The following method can experimentally determine the time τ₂ to shrinkbubble size during a cooling down state and the time τ₁ to restrict thebubble expansion to the blockage size. The experiment is performed byusing an ultra/mega sonic device coupled with a chemical liquid to cleana patterned substrate with small features of vias and trenches, wheretraceable residues exist to evaluate the cleaning performance.

A first step is to choose a τ₁ which is long enough to block thefeatures, which can be used to calculate ti, based on the equation (20).A second step is to choose different times τ₂ to run DOE. The selectionof time τ₂ is at least 10 times of τ₁, preferably 100 times of τ₁ at thefirst screen test. A third step is to fix time τ₁ and fix a power P₀ torun under at least five conditions to clean substrates with a specificpatterned structure separately. Here, P₀ is the power at which thefeatures of vias or trenches on substrate will be surely not cleanedwhen running on continuous mode (non-pulse mode). A fourth step is toinspect traceable residue status inside the features of vias or trenchesof the above five substrates by SEMS or an element analyzer tool such asEDX. The above first to fourth steps can be repeated a few times togradually shorten the time τ₂ till the traceable residues inside thefeatures of vias or trenches are observed. As the time τ₂ is shortened,the volume of bubble cannot shrink down enough, which will graduallyblock the features and influence the cleaning performance. This time iscalled critical cooling time τ_(c). After acquiring the critical coolingtime τ_(c), the time τ₂ can be set at a value larger than 2τ_(c) to havea safety margin.

A more detail example is shown as follows: a first step is to choose 10different time τ₁ as design of experiment (DOE) conditions, such as τ₁₀,2τ₁₀, 4τ₁₀, 8τ₁₀, 16τ₁₀, 32τ₁₀, 64τ₁₀, 128τ₁₀, 256τ₁₀, 512τ₁₀, as shownin Table 3 blow. A second step is to choose time τ₂ at least 10 times of512τ₁₀, preferably 20 times of 512τ₁₀ at the first screen test, as shownin Table 3. A third step is to fix a power P₀ to run under the above tenconditions to clean substrates with a specific patterned structureseparately. Here, P₀ is the power at which the features of vias ortrenches on substrate will be surely not cleaned when running oncontinuous mode (non-pulse mode). A fourth step is to use the conditionsas shown in Table 3 to process 10 substrates with features of vias ortrenches post plasma etching. The reason for choosing post plasma etchedsubstrates is that polymers generated during etching process are formedon sidewalls of trenches and vias. Those polymers formed on the bottomsor side walls of vias are difficult to remove by a conventional method.A next step is to inspect the cleaning status of features of vias ortrenches on the ten substrates by SEMS on cross-sections of thesubstrates. Resulting data are shown in Table 3 below. From Table 3, itbecomes clear that the cleaning effect reaches the best point forsubstrate #6 at τ₁=32τ₁₀, therefore the optimum time τ₁ is 32τ₁₀.

TABLE 3 Substrate# 1 2 3 4 5 6 7 8 9 10 τ₁   τ₁₀   2τ₁₀   4τ₁₀   8τ₁₀ 16τ₁₀  32τ₁₀  64τ₁₀  128τ₁₀  256τ₁₀  512τ₁₀ τ₂ 5120τ₁₀ 5120τ₁₀ 5120τ₁₀5120τ₁₀ 5120τ₁₀ 5120τ₁₀ 5120τ₁₀ 5120τ₁₀ 5120τ₁₀ 5120τ₁₀ Power P0 P0 P0P0 P0 P0 P0 P0 P0 P0 Process T₀ T₀ T₀ T₀ T₀ T₀ T₀ T₀ T₀ T₀ Time CleanStatus 1 2 3 4 5 6 5 4 4 3 of Features

If there is no peak being found, then the above first to fourth stepscan be repeated again with a wider time range of τ₁ to find the time τ₁.After finding the initial then the about first and fourth steps can berepeated again with a narrower time range τ₁ to narrow down the range oftime τ₁. After knowing the time τ_(i), the time τ₂ can be optimized byreducing the time τ₂ from 512 τ₂ to a value where the cleaning effectstarts to be reduced. A detailed procedure is disclosed as follows inTable 4. From Table 4, the cleaning effect reaches the best point forsubstrate #5 at τ₂=256τ₁₀, therefore the optimum time τ₂ is 256τ₁₀.

TABLE 4 Substrate# 1 2 3 4 5 6 7 8 τ₁  32τ₁₀  32τ₁₀  32τ₁₀  32τ₁₀  32τ₁₀ 32τ₁₀ 32τ₁₀ 32τ₁₀ τ₂ 4096τ₁₀ 2048τ₁₀ 1024τ₁₀ 512τ₁₀ 256τ₁₀ 128τ₁₀ 64τ₁₀32τ₁₀ Power P0 P0 P0 P0 P0 P0 P0 P0 Process T₀ T₀ T₀ T₀ T₀ T₀ T₀ T₀ TimeClean 3 4 5 6 7 6 5 3 Status of Features

FIGS. 21A-21C illustrates another cleaning process according anembodiment of the present invention. The present cleaning process issimilar to the one shown in FIGS. 20A-20D, with differences only in thatthe power in the present cleaning process is still on for period of mτ₁even after the cavitation reaches a saturation point Rs. Here, m can bea number between 0.1 to 100, and preferably 2, which depends on via andtrench structure and the cleaning liquid being used. And the value of mneeds to be optimized by experiment similar to the embodiment shown inFIGS. 20A-20D.

FIGS. 22A and 22B illustrate a wafer cleaning process utilizing sonicenergy according to another embodiment of the present invention. Duringthe time τ₁ when sonic power P1 is applied to cleaning liquid, bubbleimplosion starts to occur when the temperature of a first bubblereaching its implosion temperature at the point of T_(i), and then somebubble implosion continues to occur during the temperature increasingfrom T_(i) to T_(n) (during the time of Δτ). After turning off the sonicpower in the time period of τ₂, the temperature of the bubble is cooleddown from T_(n) to original T₀ by the surrounding liquid. T_(i) isdetermined as a threshold of the temperature for bubble implosion in thefeatures of vias and trenches, which triggers the first bubbleimplosion.

Since thermal transfer is not exactly uniform in the features, more andmore bubble implosion may keep occurring after the temperature reachesT_(i). The bubble implosion intensity will become higher and higherwhile the bubble temperature T increases. However, bubble implosionshould be controlled to be below the implosion intensity that wouldresult in damage to the patterned structures. Bubble implosion can becontrolled by controlling the temperature T_(n) to be below thetemperature T_(d) by adjusting time Δτ, wherein T_(n) is the bubble'smaximum temperature due to sonic power being applied to the cleaningliquid for n cycles, and T_(d) is the temperature of the accumulation ofcertain amount of bubble implosion with a high intensity (or power) toresult in the patterned structure being damaged. In the present cleaningprocess, controlling bubble implosion intensity is achieved by adjustingtime Δτ after the first bubble implosion starts, so as to achieve adesired cleaning performance and efficiency while avoiding the bubbleimplosion intensity becomes too high to cause damage to the patternedstructures under cleaning.

In order to increase particle removal efficiency (PRE), it is desirableto have a controlled transit cavitation in the ultra or mega soniccleaning process as shown in FIGS. 22A-22B. Controlled transitcavitation is achieved by setting a sonic power supply with power P₁ ata time interval shorter than τ₁, and setting the sonic power supply withpower P₂ at a time interval longer than τ₂, and repeating above stepstill the wafer is cleaned, where power P₂ is equal to zero or muchsmaller than power P₁, τ₁ is a time interval in which the temperatureinside bubble rises higher than a critical implosion temperature; and τ₂is a time interval in which the temperature inside bubble falls down toa temperature much lower than the critical implosion temperature. Sincethe controlled transit cavitation will have certain bubble implosion inthe cleaning process, the controlled transit cavitation will providehigher PRE (particle removal efficiency) with minimized damage topatterned structures. The critical implosion temperature is the lowesttemperature inside bubble which will cause the first bubble implosion.In order to further increase the PRE, it is needed to further increasetemperature of the bubbles, therefore a longer time τ₁ is needed. Alsothe temperature of bubble can be increased by shortening the time of τ₂.

The frequency of ultra or mega sonic wave is another parameter tocontrol the level of implosion. Maintaining a controlled transitcavitation can be achieved by setting a sonic power supply withfrequency f₁ at a time interval shorter than τ₁, and setting the sonicpower supply with frequency f₂ at a time interval longer than τ₂, andrepeating the above steps till the wafer is cleaned, where f₂ is muchhigher than f₁ and preferably 2 times or 4 times higher. Normally, thehigher the frequency is, the lower the level or intensity of theimplosion becomes. Again, τ₁ is a time interval during which thetemperature inside bubble rises higher than a critical implosiontemperature; and τ₂ is a time interval during which the temperatureinside bubble falls down to a temperature much lower than the criticalimplosion temperature. The controlled transit cavitation will provide ahigher PRE (particle removal efficiency) with minimized damage topatterned structures. The critical implosion temperature is the lowesttemperature inside bubble which causes the first bubble implosion. Inorder to further increase the PRE, it is needed to further increasetemperature of the bubbles, therefore a longer time τ₁ is needed. Alsothe temperature of bubble can be increased by shortening the timeinterval τ₂. Generally, an ultra or mega sonic wave with the frequencybetween 0.1 MHz-10 MHz may be applied to the wafer cleaning processesdisclosed in the present invention.

FIG. 23 illustrates an exemplary wafer cleaning apparatus for carryingout the wafer cleaning processes illustrated in FIGS. 7-22 according toan embodiment of the present invention. The wafer cleaning apparatusincludes a wafer chuck 23014 for mounting a wafer 23010. The wafer chuck23014 along with the wafer 23010 rotates during a cleaning processdriven by rotation driving mechanism 23016. The wafer cleaning apparatusalso includes a nozzle 23064 for delivering a cleaning liquid such ascleaning chemicals or de-ionized water 23060 to the wafer 23010. Anultra or mega sonic device 23062 is coupled with the nozzle 23064 forimparting ultra or mega sonic energy to the cleaning liquid. The ultraor mega sonic wave generated by the ultra or mega sonic device 23062 istransferred to the wafer 23010 through a conduit 23060 that channels thecleaning liquid to the wafer.

FIG. 24 is a cross-sectional view of another wafer cleaning apparatusfor carrying out the wafer cleaning processes illustrated in FIGS. 7-22according to an embodiment of the present invention. The wafer cleaningapparatus includes a cleaning tank 24074 containing a body of cleaningliquid 24070 and a wafer cassette 24076 holding a plurality of wafers24010 submerged in the cleaning liquid 24070. The wafer cleaningapparatus further includes an ultra or mega sonic device 24072 attachedto a wall of the cleaning tank 24074 for imparting ultra or mega sonicenergy to the cleaning liquid. There is at least one inlet (not shown)for filling the cleaning tank 24074 with the cleaning liquid 24070, sothat the wafers 24010 are submerged in the cleaning liquid 24070 duringa cleaning process.

In the above embodiments, if all the critical process parameters ofsonic power supply, such as power level, frequency, power-on time (τ₁)and power-off time (τ₂) are preset in a power supply controller withoutreal-time monitoring during a wafer cleaning process, damages topatterned structures may still occur due to some abnormal conditionsduring the wafer cleaning process. Hence, there is a need for anapparatus and method for real-time monitoring of the sonic power supplyoperation status. If the parameters are not in the normal range, thesonic power supply should be shut down and an alarm signal should besent out and reported.

FIG. 25 illustrates a control system for monitoring operation parametersof a wafer cleaning process employing sonic energy according to anembodiment of the present invention. The control system includes a hostcomputer 25080, a sonic generator 25082, a sonic transducer 1003, adetection system 25086, and a communication cable 25088. The hostcomputer 25080 sends sonic parameter settings, such as power setting P1,power-on time setting τ₁, power setting P2, power-off time setting τ₂,frequency setting, and control commands such as power enable command, tothe sonic generator 25082. The sonic generator 25082 generates sonicwaveforms after receiving these commands and sends the sonic waveformsto the sonic transducer 1003 for cleaning a wafer 1010. Meanwhile, theparameter settings sent by the host computer 25080 and outputs of thesonic generator 25082 are read by the detection system 25086. Thedetection system 25086 compares the outputs from the sonic power supply25082 with the parameter settings sent by the host computer 25080, andthen sends the comparison results to the host computer 25080 through thecommunication cable 25088. If the outputs from the sonic power supply25082 are different from the parameter settings sent by the hostcomputer 25080, the detection system 25086 sends out an alarm signal tothe host computer 25080. Upon receiving the alarm signal, the hostcomputer 25080 shuts down the sonic generator 25082 to prevent damagesto patterned structures on the wafer 1010.

FIG. 26 is a block diagram of the detection system 25086 shown in FIG.25 according to an embodiment of the present invention. The detectionsystem 25086 exemplarily includes a voltage attenuation circuit 26090, ashaping circuit 26092, a main controller 26094, a communication circuit26096 and a power circuit 26098. The main controller 26094 may beimplemented with FPGA. The communication circuit 26096 is established asan interface to the host computer 25080. The communication circuit 26096implements RS232/RS485 serial communication with the host computer 25080to read parameter settings from the host computer 25080 and sendcomparison results back to the host computer 25080. The power circuit26098 is designed to convert DC 15V to target voltages of DC 1.2V, DC3.3V and DC 5V for the whole system.

FIG. 27 is a block diagram of the detection system 25086 according toanother embodiment of the present invention. The detection system 25086exemplarily includes a voltage attenuation circuit 26090, an amplitudedetection circuit 27092, a main controller 26094, a communicationcircuit 26096 and a power circuit 26098.

FIGS. 28A-28C illustrate an exemplary implementation of the voltageattenuation circuit 26090 according to an embodiment of the presentinvention. When a sonic signal output from the sonic generator 25082 isfirst read in, it has relatively high amplitude value as shown in FIG.28B. The voltage attenuation circuit 26090 is designed to use twooperational amplifiers 28102 and 28104 to reduce the amplitude value ofthe waveform as shown in FIG. 28C. The attenuation rate of the voltageattenuation circuit 26090 is set in the range of 5 to 100, andpreferably 20. The voltage attenuation can be expressed in the followingformula:

Vout=(R2/R1)*Vin

assuming R1=200k, R2=R3=R4=10K, Vout=(R2/R1)*Vin=Vin/20

where Vout is amplitude value output by the voltage attenuation circuit26090, Vin is amplitude value input to the voltage attenuation circuit26090, and R1, R2, R3, R4 are resistances of the two operationalamplifiers 28102 and 28104.

FIGS. 29A-29C illustrate an exemplary implementation of the shapingcircuit 26092 shown in FIG. 26 according to an embodiment of the presentinvention. Referring to FIG. 26 again, the output of the voltageattenuation circuit 26090 connects to the shaping circuit 26092. Thewaveform output from the voltage attenuation circuit 26090 is an inputto the shaping circuit 26092 to convert sinusoidal wave into square wavewhich will be processed by the main controller 26094. The shapingcircuit 26092 includes a window comparator 29102 and an OR gate 29104 asshown in FIG. 29A. When Vcal−<Vin<Vcal+, Vout=0, else Vout=1, whereVcal− and Vcal+ are two threshold values, Vin is the input value of theshaping circuit, and Vout is the output value of the shaping circuit.The waveform passing through the voltage attenuation circuit 2190 is asinusoidal wave as shown in FIG. 29B. The shaping circuit 26092 convertsthe sinusoidal wave into square wave as shown in FIG. 29C.

FIGS. 30A-30C illustrate an exemplary implementation of the maincontroller 26094 of FIGS. 26 and 27 according to an embodiment of thepresent invention. The main controller 26094 includes a pulse conversionmodule 30102 and a periodic measurement module 3104 as shown FIG. 30A.The pulse conversion module 30102 is used to convert the pulse signalduring time period τ₁ to the high level signal, and keep the low levelsignal during time period τ₂ unchanged as shown in FIGS. 30B and 30C.Circuit symbols of the pulse conversion module 30102 are shown in FIG.30A, where Clk_Sys is 50 MHz clock signal, Pulse_In is an input signal,and Pulse_Out is an output signal. The periodic measurement module 30104is used to measure the time duration of the high level and low levelsignal by means of a counter using the following equation.

τ₁=Counter_H*20 ns, and τ₂=Counter_L*20 ns.

where Counter_H is the number of high level, Counter_L is the number oflow level.

The main controller 26094 compares the calculated power-on time with apreset time τ₁. If the calculated power-on time is longer than thepreset time τ₁, the main controller 26094 sends out an alarm signal tothe host computer 25080. The host computer 25080, upon receiving thealarm signal, shuts down the sonic generator 25082. The main controller26094 compares the calculated power-off time with a preset time τ₂. Ifthe calculated power-off time is shorter than the preset time τ₂, themain controller 26094 sends out an alarm signal to the host computer25080. The host computer 25080, upon receiving the alarm signal, shutsdown the sonic generator 26082. In an embodiment, the main controller26094 can be implemented using an Altera Cyclone IV FPGA model numberEP4CE22F17C6N.

FIG. 31 illustrates that the sonic power supply still oscillates severalcycles after the host computer shuts down the sonic power supply due tocharacteristics of the sonic apparatus. The time period τ₃ in which thesonic generator 25082 oscillating several cycles after power down ismeasured by the main controller 26094. This time period τ₃ can beobtained by experiments. Therefore, actual power-on time is equal toτ−τ₃, where τ is the time calculated by the periodic measurement module25104. The main controller 26094 compares the actual power-on time witha preset time T₁. If the actual power-on time is longer than the presettime T₁, the main controller 26094 sends out an alarm signal to the hostcomputer 25080.

FIGS. 32A-32C illustrate an exemplary implementation of the amplitudedetection circuit 27092 of FIG. 27 according to an embodiment of thepresent invention. The amplitude detection circuit 27092 exemplarilyincludes a reference voltage generating circuit and a comparisoncircuit. As shown in FIG. 32B, the reference voltage generating circuitis designed to use a D/A convertor 32118 to convert digital inputs fromthe main controller 26094 to analog DC reference voltages Vref+ andVref−, as shown in FIG. 27C. The comparison circuit is designed to use awindow comparator 32114 and a AND gate 32116 to compare the attenuatedamplitude Vin, an output from the voltage attenuation circuit 26090,with reference voltages Vref+ and Vref−. If the attenuated amplitude Vinexceeds the reference voltages Vref+ and/or Vref−, the amplitudedetection circuit 27092 sends out an alarm signal to the host computer25080. Upon receiving the alarm signal, the host computer 25080 shutsdown the sonic generator 25082 to avoid damaging patterned structures onthe wafer 1010.

FIG. 33 is a flow chart illustrating a wafer cleaning process accordingto an embodiment of the present invention. The wafer cleaning processbegins with step 33010 in which a cleaning liquid is applied into aspace between a wafer and an ultra/mega sonic device. In step 33020, anultra/mega sonic power supply is set at frequency f1 and power level P1to drive the ultra/mega sonic device. In step 33030, a detected power-ontime is compared with a preset time τ₁. If the detected power-on time islonger than τ₁, the power supply will be shut off and an alarm signalwill be sent out as well. In step 33040, the ultra/mega sonic powersupply is set to zero output before bubble cavitation in the cleaningliquid damaging patterned structures on the wafer. In step 33050, thesonic power supply is stored to frequency f1 and power level P1 aftertemperature inside bubble has decreased to a certain level. In step33060, a detected power-off time is compared with a preset time τ₂. Ifthe detected power-off time is shorter than τ₂, the ultra/mega sonicpower supply will be shut off and an alarm signal will be sent out aswell. In step 33070, wafer cleanliness is inspected and above steps33010-33060 will be repeated if a desired cleanliness is not met.Alternatively, inspection of cleanliness may not be performed for everycycle. Instead, the number of cycles to be used maybe empiricallydetermined beforehand using a sample wafer.

FIG. 34 is a flow chart illustrating a wafer cleaning process accordingto another embodiment of the present invention. The wafer cleaningprocess begins with step 34010 in which a cleaning liquid is appliedinto a space between a wafer and an ultra/mega sonic device. In step34020, an ultra/mega sonic power supply is set at frequency f1 and powerlevel P1 to drive the ultra/mega sonic device. In step 34030, amplitudeof the sonic power output is detected and compared with a preset value.If the detected amplitude is higher than the preset value, the powersupply will be shut off and an alarm signal will be sent out as well. Instep 34040, the sonic supply is set at zero output before bubblecavitation in the cleaning liquid damaging patterned structures on thewafer. In step 31050, the sonic power supply is restored to frequency f1and power level P1 after temperature inside the bubbles has decreased toa certain level. In step 34060, wafer cleanliness is inspected and aboutsteps 34010-34050 will be repeated if a desired cleanliness is not met.Alternatively, inspection of cleanliness may not be performed for everycycle. Instead, the number of cycles to be used maybe empiricallydetermined beforehand using a sample wafer.

In some embodiments, the wafer cleaning processes depicted in variousfigures throughout the present disclosure can be combined to produce adesired cleaning result. In one embodiment, the amplitude detection instep 34030 in FIG. 34 can be incorporated into the wafer cleaningprocess shown in FIG. 33. In another embodiment, the voltage attenuation26090 and shaping circuit 26092 of FIG. 26 as well as the amplitudedetection circuit 27092 of FIG. 27 can be applied to implement the wafercleaning processes shown in FIGS. 33 and 34.

The present invention provides an apparatus for cleaning semiconductorsubstrate using ultra/mega sonic device, comprising a chuck, anultra/mega sonic device, at least one nozzle, an ultra/mega sonic powersupply, a host computer, and a detection system. The chuck holds asemiconductor substrate. The ultra/mega sonic device is positionedadjacent to the semiconductor substrate. The at least one nozzle injectschemical liquid on the semiconductor substrate and in a gap between thesemiconductor substrate and the ultra/mega sonic device. The hostcomputer sets the ultra/mega sonic power supply at frequency f1 andpower P1 to drive the ultra/mega sonic device; before bubble cavitationin the liquid damaging patterned structure on the semiconductorsubstrate, sets the ultra/mega sonic power supply at zero output; andafter temperature inside bubble cooling down to a set temperature, setsthe ultra/mega sonic power supply at frequency f1 and power P1 again.The detection system detects power on time at power P1 and frequency f1and power off time separately, and compares the detected power on timeat power P1 and frequency f1 with a preset time τ₁. If the detectedpower on time is longer than the preset time τ1, the detection systemsends out an alarm signal to the host computer, and the host computerreceives the alarm signal and shuts down the ultra/mega sonic powersupply. The detection system also compares the detected power off timewith a preset time τ₂. If the detected power off time is shorter thanthe preset time τ₂, the detection system sends out an alarm signal tothe host computer, and the host computer receives the alarm signal andshuts down the ultra/mega sonic power supply.

In an embodiment, the ultra/mega sonic device is further coupled withthe nozzle and positioned adjacent to the semiconductor substrate, andthe energy of the ultra/mega sonic device is transmitted to thesemiconductor substrate through the liquid column out of the nozzle.

The present invention provides another apparatus for cleaningsemiconductor substrate using ultra/mega sonic device, comprising achuck, an ultra/mega sonic device, at least one nozzle, an ultra/megasonic power supply, a host computer, and a detection system. The chuckholds a semiconductor substrate. The ultra/mega sonic device ispositioned adjacent to the semiconductor substrate. The at least onenozzle injects chemical liquid on the semiconductor substrate and in agap between the semiconductor substrate and the ultra/mega sonic device.The host computer sets the ultra/mega sonic power supply at frequency f1and power P1 to drive the ultra/mega sonic device; before bubblecavitation in the liquid damaging patterned structure on thesemiconductor substrate, sets the ultra/mega sonic power supply at zerooutput; after temperature inside bubble cooling down to a settemperature, sets the ultra/mega sonic power supply at frequency f1 andpower P1 again. The detection system detects amplitude of each waveformoutput by the ultra/mega sonic power supply, and compares detectedamplitude of each waveform with a preset value. If the detectedamplitude of any waveform is larger than the preset value, the detectionsystem sends out an alarm signal to the host computer, and the hostcomputer receives the alarm signal and shuts down the ultra/mega sonicpower supply, wherein the preset value is larger than a waveformamplitude at normal operation.

In an embodiment, the ultra/mega sonic device is further coupled withthe nozzle and positioned adjacent to the semiconductor substrate, andthe energy of the ultra/mega sonic device is transmitted to thesemiconductor substrate through the liquid column out of the nozzle.

The present invention provides another apparatus for cleaningsemiconductor substrate using ultra/mega sonic device, comprising acassette, a tank, an ultra/mega sonic device, at least one inlet, anultra/mega sonic power supply, a host computer, and a detection system.The cassette holds at least one semiconductor substrate. The tank holdsthe cassette. The ultra/mega sonic device is attached to an outside wallof the tank. The at least one inlet is used for filling chemical liquidinto the tank to immerse the semiconductor substrate. The host computersets the ultra/mega sonic power supply at frequency f1 and power P1 todrive the ultra/mega sonic device; before bubble cavitation in theliquid damaging patterned structure on the semiconductor substrate, setsthe ultra/mega sonic power supply at zero output; after temperatureinside bubble cooling down to a set temperature, sets the ultra/megasonic power supply at frequency f1 and power P1 again. The detectionsystem detects power on time at power P1 and frequency f1 and power offtime separately, and compares the detected power on time at power P1 andfrequency f1 with a preset time τ₁. If the detected power on time islonger than the preset time τ₁, the detection system sends out an alarmsignal to the host computer, and the host computer receives the alarmsignal and shuts down the ultra/mega sonic power supply. The detectionsystem also compares the detected power off time with a preset time τ₂.If the detected power off time is shorter than the preset time τ₂, thedetection system sends out an alarm signal to the host computer, and thehost computer receives the alarm signal and shuts down the ultra/megasonic power supply.

The present invention provides another apparatus for cleaningsemiconductor substrate using ultra/mega sonic device, comprising acassette, a tank, an ultra/mega sonic device, at least one inlet, anultra/mega sonic power supply, a host computer and a detection system.The cassette holds at least one semiconductor substrate. The tank holdsthe cassette. The ultra/mega sonic device is attached to an outside wallof the tank. The at least one inlet is used for filling chemical liquidinto the tank to immerse the semiconductor substrate. The host computersets the ultra/mega sonic power supply at frequency f1 and power P1 todrive the ultra/mega sonic device; before bubble cavitation in theliquid damaging patterned structure on the semiconductor substrate,setting the ultra/mega sonic power supply at zero output; aftertemperature inside bubble cooling down to a set temperature, setting theultra/mega sonic power supply at frequency f1 and power P1 again. Thedetection system detects amplitude of each waveform output by theultra/mega sonic power supply, and compares detected amplitude of eachwaveform with a preset value. If detected amplitude of any waveform islarger than the preset value, the detection system sends out an alarmsignal to the host computer, and the host computer receives the alarmsignal and shuts down the ultra/mega sonic power supply, wherein thepreset value is larger than a waveform amplitude at normal operation.

While this disclosure has been particularly shown and described withreferences to exemplary embodiments thereof, it shall be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit of the claimedembodiments.

What is claimed is:
 1. A method for controlling damages in cleaning asemiconductor wafer comprising features of patterned structures, themethod comprising: delivering a cleaning liquid over a surface of asemiconductor wafer during a cleaning process; and imparting sonicenergy to the cleaning liquid from a sonic transducer during thecleaning process, wherein power is alternately supplied to the sonictransducer at a first frequency and a first power level for a firstpredetermined period of time and at a second frequency and a secondpower level for a second predetermined period of time, the firstpredetermined period of time and the second predetermined period of timeconsecutively following one another, wherein at least one of the firstand second predetermined periods of time, the first and second powerlevels, and the first and second frequencies is determined such that apercentage of damaged features as a result of the imparting sonic energyis lower than a predetermined threshold.
 2. The method of claim 1,wherein bubble sizes in the cleaning liquid increases due to sonicenergy during the first predetermined period of time and decreasesduring the second predetermined period of time.
 3. The method of claim 1further comprising rotating the wafer during the cleaning process. 4.The method of claim 1 further comprising submerging the semiconductorwafer in a cleaning tank.
 5. The method of claim 1, wherein thedelivering includes spreading the cleaning liquid through a nozzle. 6.The method of claim 1, wherein the imparting include transducing sonicenergy to a flowing clean liquid.
 7. The method of claim 1 furthercomprising reciprocally altering a distance between the sonic transducerand the semiconductor.
 8. The method of claim 1, wherein the cleanliquid is selected from a group consisting of a chemical solution,de-ionized water and a combination of the two.
 9. The method of claim 1further comprising repeating the alternation between the first andsecond periods of time for a predetermined number of times.
 10. Themethod of claim 9 further comprising empirically determining thepredetermined number of times of alternation by inspecting damages tothe semiconductor wafer.
 11. The method of claim 1, wherein the secondpower level is zero.
 12. The method of claim 1, wherein the firstfrequency is equal to the second frequency and both the frequenciesremain constant during the respective operating time period, while thefirst power level is higher than the second power level and both thepower levels remain constant during the respective operating timeperiod.
 13. The method of claim 1, wherein the first frequency is higherthan the second frequency and both the frequencies remain constantduring the respective operating time period, while the first power levelis higher than the second power level and both the power levels remainconstant during the respective operating time period.
 14. The method ofclaim 1, wherein the first frequency is lower than the second frequencyand both the frequencies remain constant during the respective operatingtime period, while the first power level is equal to the second powerlevel and both the power levels remain constant during the respectiveoperating time period.
 15. The method of claim 1, wherein the firstfrequency is lower than the second frequency and both the frequenciesremain constant during the respective operating time period, while thefirst power level is higher than the second power level and both thepower levels remain constant during the respective operating timeperiod.
 16. The method of claim 1, wherein the first frequency is lowerthan the second frequency and both the frequencies remain constantduring the respective operating time period, while the first power leveris lower than the second power level and both the power levels remainconstant during the respective operating time period.
 17. The method ofclaim 1, wherein the first power level rises during the firstpredetermined period of time.
 18. The method of claim 1, wherein thefirst power level falls during the first predetermined period of time.19. The method of claim 1, wherein the first power level both rises andfalls during the first predetermined period of time.
 20. The method ofclaim 1, wherein the second frequency is substantially close to zero andthe second power level remains a constant positive value during thesecond predetermined period of time.
 21. The method of claim 1, whereinthe second frequency is substantially close to zero and the second powerlevel remains a constant negative value during the second predeterminedperiod of time.
 22. The method of claim 1, wherein sonic waves from thesonic transducer in the first and the second periods of time haveopposite phases.
 23. The method of claim 1 further comprising detectingsonic output from a sonic generator coupled to the sonic transducer. 24.The method of claim 23, wherein the detecting includes attenuatingvoltage of an input signal.
 25. The method of claim 23, wherein thedetecting includes converting a signal from a first waveform to a secondwaveform.
 26. The method of claim 25, wherein the first waveform issinusoidal wave and the second waveform is a square wave.
 27. The methodof claim 23, wherein the detecting includes detecting and comparingamplitude of an input signal with a reference value, and causing analarm signal to be generated and the sonic generator to be turned offwhen the detected amplitude exceeds the reference value.
 28. The methodof claim 27, wherein the reference value is generated by adigital-to-analog converter (DAC).
 29. The method of claim 1, whereinthe first predetermined period of time is empirically determined toavoid bubble implosion in the cleaning liquid by inspecting thesemiconductor wafer for damages to patterned structures thereon.
 30. Themethod of claim 29, wherein the empirical determination includeschoosing different values for the first predetermined period of time indifferent cleaning processes while keeping the second predeterminedperiod of time unchanged and significantly longer than the firstpredetermined period of time, as well as keeping the first and secondfrequencies and the first and second power levels unchanged.
 31. Themethod of claim 1, wherein the first predetermined period of time isempirically determined to allow limited bubble implosions which do notcause damage to patterned structures on the semiconductor wafer undercleaning.
 32. The method of claim 31, wherein the empiricaldetermination includes choosing different values for the firstpredetermined period of time in different cleaning processes whilekeeping the second predetermined period of time unchanged andsignificantly longer than the first predetermined period of time, aswell as keeping the first and second frequencies and the first andsecond power levels unchanged.
 33. The method of claim 1, wherein thesecond predetermined period of time is empirically determined to allowtemperature of bubbles in the cleaning liquid to be cooled down to apredetermined temperature.
 34. The method of claim 33, wherein thepredetermined temperature is substantially close to a room temperature.35. The method of claim 1, wherein the first frequency and the firstpower level are empirically determined to avoid bubble implosion in thecleaning liquid by inspecting the semiconductor wafer for damages topatterned structures thereon.
 36. The method of claim 1, wherein thefirst frequency and the first power level are empirically determined toallow limited bubble implosions which do not cause damage to patternedstructures on the semiconductor wafer under cleaning.
 37. The method ofclaim 1, wherein the second frequency and the second power level areempirically determined to allow the cleaning liquid to be cooled down toa predetermined temperature.
 38. The method of claim 37, wherein thepredetermined temperature is substantially close to a room temperature.39. The method of claim 1, wherein a cleaning effect of imparting thesonic energy causes a yield improvement greater than a yield degradationcaused by damages as a result of imparting the sonic energy.
 40. Themethod of claim 1, wherein the percentage of damaged features as aresult of the imparting sonic energy is zero.